Field of the Invention
The invention lies in the mechanical arts. More specifically, the invention pertains to an anchor, preferably a plastic spreading anchor, for fixing a screw in a bore hole, such as a drill hole or a throughhole. The anchor is formed with several longitudinal spreading elements between a neck portion and a head portion, which is provided with an annular collar. The screw is insertible through the neck portion.
In such prior art anchors a guide hole extends through the center of the head for the purpose of providing a self-tapping guide, i.e., into which the screw cuts its own thread. With increasing rotation of the screw, the head is pulled in a direction towards the wall surface in which the bore hole is formed. Due to the approach between the head and the neck portion, the spreading elements are expanded, until they come to lie against the wall of the bore hole and thereby engage in a form-locking and/or force-locking connection.
In solid building materials, such as hard concrete or natural rock, this leads to only a slight rotation of the head and/or to a twisting of the spreading elements. In softer materials on the other hand, the rearwardly moving head rotates relative to the rotationally stationary neck portion and thus twists the spreading elements.